This invention relates to an improved contactless electrical energy transmission system in which a transformer provides the only coupling between the power transmitter and the power receiver, and more particularly to an energy efficient system which tightly regulates the output against the input voltage and output current changes.
In many applications, the contactless electrical energy transmission (CEET) has distinct advantages over the conventional energy transmission system which uses wires and connectors. For example, the CEET has been the preferred power-delivery approach in hazardous applications such as mining and underwater environments due to the elimination of the sparking and the risk of electrical shocks [1]. Also, a number of CEET systems have been developed for electric vehicle battery-recharging applications because of their potential enhanced safety, reliability, and convenience. In addition, the CEET has been considered in medical applications since it makes possible to transfer electric energy, which is required for running implanted electrical circulatory assist devices, through the intact skin of a patient [2]. Finally, the CEET has been used in cordless electric toothbrushes and portable telephones to increase their reliability by eliminating the contacts between their battery charger and the battery.
Generally, the CEET is implemented by using magnetic induction, i.e., by employing specially constructed transformers. In such transformers, the energy from the primary to the secondary is transferred inductively through the air. Because of safety requirements and/or mechanical constraints, the transformers for CEET applications usually have a relatively large separation between the primary and secondary winding. Therefore, the characteristics of these transformers are very different from those of the conventional transformers which have good coupling between the windings.
Due to a large winding separation, the transformers for CEET applications have a relatively large leakage inductance, as well as increased proximity-effect winding losses. Furthermore, for the CEET transformers where the primary and secondary winding are wound on two magnetic pieces separated by an air gap, the magnetizing inductance is significantly reduced. This reduced magnetizing inductance increases the conduction losses because of the increased magnetizing current. In addition, due to a strong radiation from the gap, the transformers with a large air gap require a special attention to meet EMC requirements. To alleviate problems associated with the energy stored in a relatively large leakage inductance of a CEET transformer such as a reduced efficiency and increased component stress, converter topologies which incorporate the leakage inductance into the circuit operation such as resonant and soft-switched topologies are the optimal choice in CEET applications.
A typical CEET system consists of a transmitter, CEET transformer, and receiver, as shown in FIG. 1. The transmitter function is to generate an ac signal that is transferred to the receiver through the transformer. The transmitted signal is then conditioned by the receiver to provide the desire signal to the load. Generally, output voltage V.sub.o of the CEET can be either a dc or an ac voltage. Similarly, the input to the CEET system can be either a dc voltage, as shown in FIG. 1, or an ac voltage, as shown in FIG. 2. As can be seen from FIG. 2, for the ac input, the rectified input voltage is applied to the input of the transmitter.
With no connections between the input side and output side, the control and protection of CEET converters is very much different than the control of the converters that employ a conventional feedback control that uses signal communication between the output and input. Moreover, in CEET applications with wide input-voltage and load ranges that require a tight regulation of the output such, for example, in universal-line (90-264 Vac) chargers for portable telephones, control design requires a unique approach.
Various aspects of inductive CEET have been addressed in a number of papers and patents. For example, mechanical design issues related to CEET systems were discussed in [1], [3]-[7], where a number of mechanical structures for CEET systems were proposed. The common goal of all these inventions is to define a reliable, easy-to-use mechanical structure which can provide consistent characteristics of the CEET transformer so that the conversion efficiency and EMC performance can be optimized.
Some limited topological and control issues were presented in [1], [3], and [8]. In [1] and [3], inductive coupling was implemented at a high-frequency by employing an inverter on the input side to convert the source signal into a high-frequency signal that is coupled to the output rectifier circuit through a CEET transformer. In addition, [1] proposes a method of maximizing the transmitted power by the input-side control employing a phase-locked loop. In [8], a line-frequency inductive-coupling scheme and output voltage control using a saturable output-side inductor are described. Generally, a line-frequency CEET suffers from a significantly larger size and weight compared to its high-frequency counterpart.
However, so far, no high-power density, high-efficiency CEET system with tightly regulated output that is suitable for applications with wide input and load range has been proposed. Namely, the known CEET approaches are implemented so that power flow from the primary (input) to the secondary side (output) of the transformer is controlled. In addition, the regulation of the power flow is performed only on one side, i.e., either on the transmitter (input), or on the receiver (output) side. Therefore, it is hard to simultaneously optimize the conversion efficiency and achieve a precise output regulation under widely varying input voltage and load conditions. For example, if the load demand is decreased, a CEET system with a controllable transmitter can reduce the transferred power through the transformer either by reducing the input power, and/or by returning the excessive power stored in the primary-side resonant circuit to the input. However, it cannot precisely regulate the output because of the absence of direct communication link between the output and the controller, i.e., because it cannot control the energy transferred to the secondary side. On the other hand, a CEET system with a controllable receiver can precisely regulate the output with a local feedback loop. However, it cannot reduce the input power because of the absence of control in the transmitter. As a result, this type of CEET system must store the excessive energy in energy-storage components, which usually increases the stress on the components. In both cases, the conversion efficiency is adversely affected.
A CEET approach which can simultaneously achieve high efficiency and precise voltage regulation must be implemented with a topology which allows a controlled bi-directional power flow through the transformer, and should have a local regulation in both the transmitter and the receiver. With this approach, the operation of the CEET system with a wide input range and a wide load range can be achieved with a minimum circulating energy and stress on the components. Consequently, this approach exhibits a good efficiency and output regulation.